In a variety of semiconductor applications it is desirable to access buried semiconductor layers in order to oxidize the buried layer. Numerous photonic, electronic and micro-mechanical devices require a region of buried material that is electrically insulating or differs from surrounding material by having a lower refractive index. Such a material can be formed during device fabrication by selectively converting one or more buried semiconductor layers into an electrically insulating low refractive index native oxide.
An oxidized region may be made to partially or completely surround a region of interest to produce a desired optical effect due to the lower refractive index of the oxide region. Buried oxide layers may also function to electrically isolate different regions of a device or to electrically isolate one device from another on the same wafer. The buried layer material may be any material that oxidizes rapidly in a lateral direction and is typically a semiconductor having a high aluminum content such as AlGaAs, AlGalnP or AlAsSb. The buried layer may be, but is not limited to compounds containing aluminum and one or more of the following elements: As, Ga, In, P and Sb. Aluminum will typically comprise at least 70% of the Group III component of the compound. Exposure to an oxidizing environment such as steam at elevated temperature the buried layer would oxidize laterally, proceeding from exposed sidewalls inward towards unoxidized portions of the material. The lateral oxidation rate generally increases with increasing aluminum content.
The usual method of accessing the buried oxidation layers is through a mesa etch. This method leads to a high level of wafer non-planarity that complicates subsequent processing steps. Moreover, the large amount of materials removed degrade the device""s mechanical integrity and increases its thermal resistance. The problem is especially severe in devices like vertical-cavity surface-emitting lasers, where the oxidation layer is usually embedded far beneath the wafer surface.
Instead of forming mesas that expose the sidewalls of the layers to be oxidized, a plurality of etched cavities may be used to access, the buried layer for oxidation. The shape and size of the resulting oxidized region are defined by the shape of each cavity and by placement of the cavities with respect to each other. The area between cavities remains planar, which eases further processing steps such as electrical contact formation and photolithography. Planar structures allow for simple etch, deposition, photolithography steps without concern for depth of focus issues during photolithography or problems with step coverage during deposition or spin coating of dielectric and or polymer films. The materials between etched cavities remain intact, so good mechanical integrity and thermal conductivity can be maintained.
Planar lateral oxidation benefits a number of applications. Applications include but are not limited to applications containing a core region that is surrounded by buried oxidized materials, where the oxidized materials provide optical waveguiding, a defined electrical conduction path, or both; applications where one or more completely oxidized layers are used for their optical filtering properties; applications that employ the oxidized layers for electrical isolation; and applications where properties are controlled by the shape of the oxidized region.